Now we are ready to give a formal theorem in the style of [1]} for the acoustic equations formulated as
\(\left\lbrace \begin{aligned}\partial _t \sigma +\frac{2}{3}(\overline{K}_A-1)\nabla _x\cdot u&=0,\\\partial _t u +\nabla _x(\sigma +\theta )&=0,\\\partial _t\theta +\frac{2}{3}\nabla _x\cdot u&=0,\end{aligned}\right.\) 
